The present invention relates to a device for the optical recording, storage and readout of information with at least one memory element in which interference patterns can be stored and/or which possesses a light diffracting structure and/or in which a light diffracting structure can be generated, means to generate at least two light rays, with at least two of the light rays being able to be aligned in such a way that they can be combined in the memory element while forming an interference pattern, means for the frequency control or frequency shift and/or frequency modulation or for phase shift and/or phase modulation, by means of which at least one of the light rays guided to the memory element can be modulated, with at least one detector to detect the light rays and with at least one demodulator and/or with optical elements, by means of which a time and/or space modulation of the intensity with reference to the whole or parts of the detected ray cross-section can be measured.
The invention further relates to a device for the optical readout of information with at least one memory element in which interference patterns can be stored and/or which possesses a light diffracting structure and/or in which a light diffracting structure can be generated, means to generate at least one light ray on the memory element, with at least one other light ray being able to be generated with the memory element and with at least two light rays being able to be aligned in such a way that they can be combined while forming an interference pattern, means for frequency control and/or frequency shift and/or frequency modulation or for phase shift and/or phase modulation by means of which at least one of the combined light rays can be modulated, with at least one detector to detect the light rays and with at least one demodulator and/or with optical elements by means of which a time and/or space modulation of the intensity with reference to the whole or parts of the ray cross-section detected can be measured.
The invention further relates to the use of the device in accordance with the invention for the optical recording, storage and/or readout of information.
The invention further relates to a method for the optical recording of information in which in a memory element possessing a light diffracting structure and/or in which light diffracting structures can be generated, at least two light rays can be combined while forming an interference pattern for the purpose of the recording and a method for the optical display of information from a memory element possessing light diffraction structures in which memory element for the purpose of detecting a light diffracting structure at least two light rays are combined while forming an interference pattern with the frequency or the phase position of at least one of the light rays being varied/and or modulated over the others and with at least one of the light rays being emitted from the memory element and being detected by a detector and with a time or space modulation of this light ray being demodulated by means of a demodulator.
Optical memories are currently normally realized as page oriented holographic memories (POHM). Currently favored is a conventional optical array in a 90 degree geometry to record Fourier transform holograms and the angle multiplex method for the selection of the individual holograms. A pattern stored as a hologram at a certain angle represents one page, the individual picture elements of the pattern (pixels) represent the information stored on this page. The number of pages which can be stored in the angle multiplex process is limited by the angle resolution of the individual holograms and by the diffraction efficiency which decreases roughly inversely proportionally to the square of the number of holograms to be stored. The current developments therefore aim for the highest possible storage capacity per page.
FIG. 1 shows the principal design of a POHM. The beam splitter 10 splits an incident coherent light ray into a "reference" and an "object" ray. The reference ray can be switched by means of the shutter 12 and is guided to the memory element 30 via a mirror 20. The object ray can be switched by shutter 14 and is guided to a spatial light modulator (SLM) 40 via a mirror 22. The SLM 40 represents the information to be stored on one side, i.e. as an individual hologram, as a pattern of light and dark picture elements (pixels). This pattern is imaged by the lenses 50 and 52 through the memory element 30 true to size on a camera, normally a charge coupled device (CCD) 60. The pattern obtained at SLM 40 is thus restored by the CCD 60.
To record a page, the memory element 30 is turned to a suitable position, the information to be stored is presented at the SLM 40 and both shutters 12, 14 are opened for a suitable period (exposure time). The resulting interference pattern in the memory element 30 is stored as a hologram. Different holograms can be stored at different angle of rotation positions. To read a certain page, the memory element is turned into the angle of rotation position used in the recording and only shutter 12 is opened, i.e. the reference ray is applied. The pattern obtaining at the SLM 40 at the time of recording is then restored by the hologram and imaged on the CCD 60 where it can be accessed.
To improve the signal to noise ratio during the reading of the holograms, a heterodyne detection process for holographically optical memories has already been proposed. In this process, instead of the CCDs 60, a detector array has to be used whose single elements (pixels) react by means of suitable demodulators only to a time modulation of the intensity of the corresponding pixel. In the reading process, the image obtaining at the detector array is overlapped by an additional flat wave with a suitably different frequency over the reference ray. The resulting interference shows the required time modulation of the individual pixels.
With the generic optical memories, it is disadvantageous that the optimization of the memory capacity requires a substantial effort with regard to the quality of all optical components and of the memory medium due to an increase in the number of pages and in particular to the capacity per page. The number of pages which can be stored in the angle multiplex method is limited by the physical situation, heterodyne detection processes being unsuitable for a large number of pixels per page due to the technical effort to be effected. It is therefore the object of the present invention to provide a device and a method for the optical recording, storage and display of date in which a high memory capacity is possible with relatively low demands on the optical quality of the components used.